1. Field of the Invention
This invention relates to a cylinder-injection spark-ignition internal combustion engine, and more particularly, to a control apparatus for controlling an automotive internal combustion engine of this kind.
2. Related Art
In a spark-ignition internal combustion engine mounted on a vehicle, various types of cylinder-injection gasoline engines, in which fuel is directly injected into a combustion chamber, have been recently proposed to reduce emission of harmful exhaust gas components and improve the fuel efficiency, instead of a conventional manifold-injection engine, in which fuel is injected into an intake pipe.
A cylinder-injection gasoline engine is arranged to inject fuel from a fuel injector into a cavity formed at a top face of a piston, to thereby supply an air-fuel mixture having an approximate stoichiometric air-fuel ratio around a spark plug at the time of ignition, whereby ignition is enabled even if a mixture as seen in the entirety of a cylinder has a lean air-fuel ratio, so that the emission of CO and HC is reduced and the fuel efficiency at the time of idle operation or low load traveling is largely improved.
In such a gasoline engine, a shift is made between a compression-stroke injection mode (second-term injection mode) and an intake-stroke injection mode (first-term injection mode) in dependence on engine operating condition or engine load. More specifically, at the time of low load operation, fuel is injected in the compression stroke so that an air-fuel mixture having an approximate stoichiometric air-fuel ratio is formed around the spark plug or in the cavity, thereby enabling excellent ignition with a mixture whose air-fuel ratio is as a whole lean. On the other hand, at the time of medium or high load operation, fuel is injected in the intake stroke so that a mixture whose air-fuel ratio is uniform in the combustion chamber is supplied, whereby a large amount of fuel is burnt to produce an engine output required at the time of acceleration or high speed traveling, as in the case of a conventional manifold-injection type gasoline engine.
Japanese Unexamined Patent Publication No. 5-99020 discloses, at the introduction part of the specification, a 2-cycle cylinder-injection internal combustion engine, as a prior art, in which a fuel injection amount at the time of low load operation is calculated depending on a throttle valve opening and engine rotation speed and in which a fuel injection amount at the time of engine high load operation is calculated depending on intake air amount detected by an air flow meter and engine rotation speed. In this internal combustion engine, when the throttle valve opening is changed, not only the fuel injection amount is calculated and adjusted, but also the intake air amount supplied into the cylinder is adjusted. For the intake air amount adjustment, the opening degree of an air control valve is controlled, which valve is provided in a bypass line, bypassing a mechanical supercharger disposed in the intake pipe of the engine.
In this 2-cycle cylinder-injection engine, a delay occurs between when the throttle valve opening is changed and when the intake air amount supplied to the cylinder reaches a required amount, which is determined by the changed throttle valve opening and the engine rotation speed. On the other hand, the cylinder-injection internal combustion engine can supply, without a delay, the cylinder with fuel in the same amount as a calculated fuel injection amount when the calculated amount changes with a change in the throttle valve opening, as distinct from the internal combustion engine, in which fuel is injected into the intake pipe. In this regard, the aforementioned 2-cycle engine entails a problem that an actual air-fuel ratio is deviated from an optimum air-fuel ratio until the intake air amount supplied to the cylinder reaches a required amount determined by the changed throttle valve opening and the engine rotation speed.
To eliminate such a problem, the aforesaid Japanese Patent Publication proposes a technical art, in which, at the time of calculating the fuel injection amount based on throttle valve opening, a response of a fuel injection amount change to a change in throttle valve opening is retarded than a response, at the time of fuel injection amount calculation based on intake air amount, of a change in the fuel injection amount to a change in the intake air amount. More specifically, a filtering quantity for the throttle-valve-opening-based control is set to be larger than that for the intake-air-amount-based control.
In detail, according to the technical art described in the above Japanese Patent Publication, in addition to the air control valve provided in a bypass line, bypassing a mechanical supercharger disposed in the intake air pipe at a location downstream of the throttle valve, an air bypass valve is provided in another bypass line, which bypasses the throttle valve. To eliminate the problem that an optimum intake air corresponding to the fuel injection amount cannot be supplied to the cylinder, even if the intake air amount is controlled by the throttle valve, at the time of low-load operation in which the fuel injection amount increases with an increase of the accelerator pedal depressing amount, the openings of the air control valve and the air bypass valve are adjusted in the throttle-valve-opening-based fuel injection amount control to obtain an optimum intake air amount suited to the fuel injection amount and prevent an occurrence of a large difference between pressures at locations upstream and downstream of the supercharger to thereby suppress a driving loss of the mechanical supercharger. As a consequence, an amount of air returned to the upstream of the mechanical supercharger through the bypass lines is adjusted. Further, a filtering quantity is increased in the throttle-valve-opening-based fuel injection amount control, to thereby eliminate a delay in response in the air amount adjustment.
However, no clear relation is found between the fuel injection amount and the intake air amount in case that a control of the fuel injection amount is made while adjusting intake air amount, as in the case disclosed in the Japanese Patent Publication. For this reason, it is difficult to obtain an intake air amount suited to the fuel injection amount, so that a sufficient engine output cannot be obtained or a combustion state can be worsened. If a deteriorated combustion state is left, resultant harmful gases are discharged from the engine to the atmosphere, and a deterioration of the engine is caused.
In a typical cylinder-injection gasoline engine, a shift is made between a first- and second-term injection modes in dependence on engine load, as described above. In the first-term injection mode, the air-fuel ratio cannot be made too lean, and hence the air-fuel ratio is set to a value of about 20 or less. On the other hand, in the second-term injection mode where the fuel is injected in a latter stage of the compression stroke, the degree of stratification of an air-fuel mixture is high and an approximate stoichiometric air-fuel mixture is formed locally around the spark plug. If the air-fuel ratio is adjusted to a value on an excessively fuel-rich side, then a misfire may be caused in the engine. Usually, therefore, the air-fuel ratio is set to a value of about 22 or more. As a result, an air-fuel ratio range in which combustion is disabled is present between the air-fuel ratio of 20 and 22.
The combustion-disabled range is inevitably passed when a changeover is made between the first- and second-term injection modes. In the combustion-disabled range, the operating state of the engine is worsened and the engine output torque temporarily decreases or increases. If even a temporal increase or decrease in the engine output torque occurs at the time of mode changeover, an undesired torque shock is caused.
Japanese Unexamined Patent Publication No. 63-12850 states that, in case that a target air-fuel ratio for a conventional manifold-injection engine is changed in accordance with intake pipe pressure, a change rate of engine rotation speed (or change rate of vehicle speed), and throttle opening, if the same changeover speed is used between when the target ratio is switched from the stoichiometric air-fuel ratio to a lean air-fuel ratio and when the target ratio is switched from a lean air-fuel ratio to the stoichiometric air-fuel ratio, an undesired large shock occurs or an amount of NOx emission increases at the time of changeover of the target air-fuel ratio. To obviate this, the technical art disclosed in the just-mentioned Japanese Patent Publication causes the changeover speed, at the time of changeover to a lean air-fuel ratio, to be lowered to give a high priority to a reduction of shock in view of the fact that a large shock occurs and a NOx emission level is high when a shift is made from the stoichiometric air-fuel ratio to a lean air-fuel ratio. On the other hand, when a lean air-fuel ratio is switched to the stoichiometric air-fuel ratio, the changeover speed is increased to give a high priority to a reduction of NOx emission in view of the fact that a shock is relatively small and the NOx emission level is low and gradually decreases with an increase in the changeover speed.
However, it is difficult to apply the technical art, described in the above Japanese Patent Publication and constructed to meet a manifold-injection engine, to a cylinder-injection engine, in which fuel injection timing is changed upon changeover of injection mode and in which the air-fuel ratio passes through a combustion-disabled range. Moreover, even if the technical art is applicable to an cylinder-injection engine, it is impossible to ensure an appropriate combustion state and reduce a torque shock in a cylinder-injection engine, which is entirely different in engine characteristics and in control method from the intake-manifold injection engine.